On an aircraft, such as single main rotor helicopter, speed is limited by asymmetry of lift on the rotor, which creates stall on the retreating side and high power requirements on the advancing side. The X2 TECHNOLOGY™ concept/platform overcomes this restriction in speed with co-axial counter-rotating rotors in which the advancing side of each rotor provides lift and the retreating side blades are offloaded.
The co-axial configuration overcomes traditional limits on helicopter speed. However, other factors serve to limit the top speed of the co-axial configuration. These other factors include installed power, vibratory limits, and pitch stability.
Conventional techniques for improving pitch stability focus on an increase in tail volume, improved tail effectiveness, or control of aircraft center of gravity (CG). Increasing tail volume can increase the aircraft weight, tail loads, and footprint. Moreover, increasing tail volume results in undesirable tail forces for some flight conditions and maneuvers. Control of CG may be desirable from a stability perspective, but subject to many design constraints and so there will often be a limited ability to modify CG. Also, for aircraft with a canted tail rotor or lifting tail, it is desirable from a performance perspective (especially at hover) to have the CG aft of the shaft, which is bad for stability on rigid rotor aircraft.